Transducers generally convert electrical signals to mechanical signals or vibrations, and/or mechanical signals or vibrations to electrical signals. Acoustic transducers, in particular, convert electrical signals to acoustic signals (sound waves) and convert received acoustic waves to electrical signals via inverse and direct piezoelectric effect. Acoustic transducers generally include acoustic resonators, such as thin film bulk acoustic resonators (FBARs), surface acoustic wave (SAW) resonators or bulk acoustic wave (BAW) resonators, and may be used in a wide variety of electronic applications, such as cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers and other portable communications devices. For example, FBARs may be used for electrical filters and voltage transformers. Generally, an acoustic resonator has a layer of piezoelectric material between two conductive plates (electrodes), which may be formed on a thin membrane. FBAR devices, in particular, generate longitudinal acoustic waves and lateral (or transverse) acoustic waves when stimulated by an applied time-varying electric field, as well as higher order harmonic mixing products.
Stacked bulk acoustic resonators (SBARs) include a second FBAR stacked upon a first FBAR. A type of SBAR characterized as a double FBAR (DBAR) may include two layers of piezoelectric material with three electrodes in a single stack. That is, a first layer of piezoelectric material is formed between a bottom electrode and a middle electrode, and a second piezoelectric material is formed between the middle electrode and a top electrode. Generally, DBARs allow reduction of the area of a single bulk acoustic resonator device by about half Examples of SBARs and FBARs, as well as their materials and construction, may be found in U.S. Pat. No. 7,367,095 to Larson, III et al., published May 6, 2008, which is hereby incorporated by reference.
There is an increasing demand to extend the frequency range of acoustic resonators for use in compact devices such as cellular phones for example. What is needed therefore is structure and construction method that extends the resonance frequency of acoustic resonators for both high frequency and low frequency applications.